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Hawaiian Islands Synoptic Discussion and Guidance

341
FXHW60 PHFO 220632
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
832 PM HST Sat Apr 21 2018

.SYNOPSIS...
High clouds moving northeastward within the subtropical jet
stream will cover Maui and the Big Island tonight and Sunday.
Trade showers are expected to decrease overnight, leading to drier
weather across windward and mauka areas for Sunday. Another area
of low level moisture will increase trade showers Monday night
into Tuesday. Trade winds will become locally windy Monday into
Tuesday, then slowly decline later in the week. A cold front may
push down the chain with clouds and showers by next weekend.

&&

.DISCUSSION...
Trade wind weather will continue for the next several days as a
strong surface high passes far north of the islands on Monday.
This high will strengthen trade winds to locally windy levels
across the area Monday through Tuesday. Wind speeds will
gradually decrease after that and trade flow may possibly become
disrupted starting Friday as a surface low passes far to the
north. Models disagree on whether or not we will maintain light
trades on Friday or if local diurnal winds will take over. By
Friday night and Saturday, models show a late-season frontal
boundary passing down the chain, with breezy trades returning
afterwards.

Thick middle and high clouds, associated with the subtropical
jet, continue to cover Maui County and the Big Island, with Oahu
and Kauai on the northern edge. Models keep this area around through
Sunday. At lower levels, the back edge of an area of scattered to
broken clouds and showers riding in on the trades is approaching
the Big Island. This should lead to fewer windward and mauka
showers later in the night and into Sunday for most of the state.
Both GFS and ECMWF show another trade wind shower area arriving
across windward Maui and the Big Island late Monday night or
Tuesday. However, this week looks to be fairly quiet. Shower
timing and placement on Friday will depend on how much trade flow
is disrupted. However, expect an increase in showers Friday night
into Saturday associated with the frontal boundary.

&&

.AVIATION...
A high pressure ridge north of the islands will continue to
produce moderate to locally strong trade winds across the
islands. An upper level jet stream will continue to produce
moderate turbulence over the islands this evening. A stable
atmosphere will keep VFR conditions in effect for most areas with
lower ceilings and periods of scattered showers expected along
windward slopes of each islands.

AIRMET Sierra is in effect for tempo mountain obscuration over
windward sections of Oahu, Molokai, Maui and the Big Island.
Conditions are expected to remain in place overnight for most
areas.

AIRMET Tango remains in effect for moderate low level turbulence
over and downwind of mountain ranges for all islands due to the
moderate to breezy trade winds. Expect these conditions to
continue through at least Sunday.

Models show an area of moderate to severe upper level clear air
turbulence over the islands, which is covered under SIGMET UNIFORM
series. The forecast shows this area sticking around into Sunday,
but weakening to moderate levels overnight. As this happens the
SIGMET will be able to be dropped and could prompt another AIRMET
Tango for moderate upper level turbulence over at least part of
the local area.

&&

.MARINE...
Fresh to strong trades are forecast to hold through much of the
upcoming week as high pressure remains established to the north.
The small craft advisory currently in place for most marine areas
from Oahu to the Big Island will hold through Sunday. This
advisory will likely be extended and potentially expanded to other
waters Monday through Wednesday as high pressure strengthens and
a moderate northwest swell builds down the island chain boosting
seas up to around the 10 ft mark.

Rough surf is forecast to persist through the week along east facing
shores due to fresh to strong trades holding in place locally and
just upstream of the area. The best chance for advisory-level surf
will come Tuesday through Thursday as the trades increase locally
and expand farther upstream in response to 1030-1035 mb high
building to the north. Although confidence remains low this far out
in the forecast, there could be a break in the trades by Friday
as a front moves into the area.

A new northwest (320 deg) swell is expected to slowly fill in late
Sunday afternoon, peak Monday, then slowly ease through midweek.
This source is from a potent system that reached storm-force
category Friday as it crossed the date line just south of the
Aleutians between 40N and 50N (around 2000 miles northwest of the
state). Surf is forecast to peak on Monday at heights below advisory
levels along north and west facing shores.

A reinforcing moderate northwest (330 deg) swell is forecast to fill
in Thursday, peak Thursday night into Friday, then ease into the
weekend. This reinforcement will be from a developing storm that is
currently near the Kamchatka Peninsula and forecast to track east
while skirting the Aleutian Islands Sunday through Wednesday.
Surf associated with this feature should remain below advisory
levels along north and west facing shores.

A reinforcing southwest (220 deg) swell is forecast to fill in
Monday and hold through midweek due to recent activity across the
southern Pacific/Tasman Sea. Small to moderate surf will result
along south facing shores each day.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST Sunday for Oahu Leeward
Waters-Kaiwi Channel-Maui County Windward Waters-Maui County
Leeward Waters-Maalaea Bay-Pailolo Channel-Alenuihaha Channel-
Big Island Leeward Waters-Big Island Southeast Waters.

&&

$$

DISCUSSION...Powell
AVIATION...Bohlin
MARINE...Gibbs

Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office

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RH		19 %
Wind		NE 13 mph
Road		Open (4x4)